The Arizona Cardinals called with such an opportunity, and the 22-year-old rookie defensive tackle is making the most of his time in training camp thus far.

"Bruce Gaston did a heck of a job on our defensive line -- really showed up, blowing up some blockers and made a little name for himself in that scrimmage (Saturday)," said head coach Bruce Arians, who later told Bickley and Marotta on Arizona Sports 98.7 FM that Gaston "is a big, powerful guy. I mean when I saw a couple of offensive lineman come backwards, it's just like, 'Whoa, who's over there?' I thought it was (Darnell Dockett) or Calais (Campbell) or somebody, and it was (No.) 75. I said, 'Hmm, I like that.'"

Gaston, listed at 6-foot-2 and 310 pounds -- in other words, a big body in the middle of the defensive line -- had not heard of Arians' praise. When he did, he was appreciative.

"But my main thing right now is just to continue to learn the playbook and just continue to get better, do what (defensive line) Coach (Brentson) Buckner asks of me and try to be a dominant defensive lineman," he said. "That's my main goal right now."

Gaston's play has spoke volumes, and with Dan Williams slowed by a injured left knee and Alameda Ta'amu still rehabbing a torn ACL, the rookie finds himself running with the first-team defense -- with Dockett to his right and Campbell to his left. Gaston called playing with first stringers "crazy" and "mind-blowing."

"Coach Buck tells us to be critical when you watch film of yourself," he said. "That's one thing I'm really trying to do -- is be critical of myself. So the next practice or the next day, I really need to focus on those things."

Making it in the NFL is hard enough on a rookie, let alone a rookie who goes undrafted. When the 2014 NFL Draft ended, 256 names had been called, and not one of them was Gaston's. In four yeas as a defensive tackle at Purdue, he totaled 131 tackles (24 for loss), seven sacks, four fumble recoveries, two forced fumbles and two passes defensed in 50 games (including 44 starts).

"Obviously, I was very disappointed, extremely disappointed," he said. "But I do believe everything happens for a reason, whether we agree with it or not, and I know a lot of times we don't always agree with it. Even now, I'm still not necessarily agreeing with that decision, but it happened. So, once it happened, all I could do is move forward."

Gaston was one of 15 rookie free agents the Cardinals signed following the draft.

"Once I got here," he said, "it's like, 'Okay, it's over with.' I've just got to strive to be better and be the best and continue to grow. So, from draft weekend to now, I use that as motivation. My goal is (to) prove everybody wrong, show them that, 'Ah, you know what, we should've drafted this cat. Dang, we missed out.' That's something I use as motivation as well."

It's still too early to determine whether or not Gaston will make the 53-man roster, but it's a good bet he'll see a considerable amount of work in the preseason, which begins Saturday against the Houston Texans at University of Phoenix Stadium.

"Tenacity and just hard work, and try to be as physical as I can be," he said when asked what he's shown the coaching staff that's resulted in his rise up the depth chart. "I want to be a nasty defensive lineman. They always say that in the meeting rooms: We've got to be a little bit more nastier, a little bit more vicious. I'm just trying to apply that to my game."